Lithium ion rechargeable batteries, which are a nonaqueous electrolyte rechargeable battery, are widely used in portable electronic devices which have been made smaller, lighter, and more powerful, such as video cameras, portable audio players, mobile phones, and notebook computers. For such applications, there is a demand for lithium ion rechargeable batteries having still higher capacity and improved load characteristics. In industrial production, it is necessary to manufacture the products which fulfill the demand in stable quality.
In order to meet the demand, Patent Publication 1 proposes a cathode active material having a large capacitance per unit volume, good safety, and excellent cycle characteristics. This cathode active material is generally in a spherical form, and a mixture of larger diameter particles having a mean particle diameter of 7 to 20 μm and smaller diameter particles having a mean particle diameter of 10 to 30% of that of the larger diameter particles, at a ratio of 1:2 to 9:1 by mass.
Patent Publication 2 proposes a cathode active material excellent in load characteristics and cycle characteristics and high in capacity. This cathode active material contains a larger particle diameter group having a particle diameter of 15 to 22 μm and a smaller particle diameter group having a particle diameter of 1 to 6 μm, and the ratio of the smaller particle diameter group to the larger particle diameter group is 0.25 to 0.6 by weight.
Patent Publication 3 proposes a cathode active material which may improve the current collection efficiency and is excellent in high rate performance and cycle characteristics. This cathode active material has secondary particles having a mean particle diameter of 1 to 10 μm formed by aggregation of 1 to 20 primary particles, an angle of repose of not larger than 60°, and a specific surface area of 0.5 to 1.0 m2/g.    Patent Publication 1: JP-2004-119218-A    Patent Publication 2: JP-2002-93417-A    Patent Publication 3: JP-2003-288899-A
In producing electrode slurry with the above-mentioned cathode active materials, homogeneous dispersion of the cathode active material, an electrically conductive material, and a binder is hard to be achieved, the fluidity may not be increased, and electrode slurry having little change in dispersity and fluidity over time is hard to be produced. Thus, particularly in industrial production, electrode slurry cannot be applied to electrode plates under constant conditions, so that electrodes of stable quality cannot be produced. For example, when an electrode slurry is applied to electrode plates, the dispersity of the electrically conductive material or the amount or the particle size distribution of the cathode active material in the electrode soon after the application will change a few hours after the application, resulting in electrodes having different load characteristics or capacities.
Therefore, it is an object of the present invention to provide a cathode active material for nonaqueous electrolyte rechargeable batteries which allows production of batteries having improved load characteristics in stable quality, and also allows production of batteries having high capacity, as well as a cathode for nonaqueous electrolyte rechargeable batteries produced with this cathode active material, and a nonaqueous electrolyte rechargeable battery produced with this cathode.